Have any Forum members looked carefully at the relative energy, $ cost, and tradeoffs of solar thermal hot water systems versus the alternatives for vessels (like us) not on shorepower and at anchor or moored, not underway most of the time (and therefore not using the motor heat exchangers to heat the hot water system (HWS)?
Ocean Alexander Aquabelle has motors and AC from generator to heat a 1.8kWe element in an 81 liter HWS tank in the engine room. But most of the time, we don't use the motors as we are moored. Our current options are:
To go from 10 to 70 degrees C takes about 4-5 kWhe and about 1.5 hours running the genny which uses about 4+ liters of diesel--a highly inefficient use of the fuel when heating water is the primary reason to run the genny.
: Aquabelle now has 2.4 kWe of solar which generates substantially more power on a sunny day than needed for house use. We can switch on the HWS circuit breaker to heat the HWS via the inverter but a 50A solar current immediately shifts to a -70 to 80A drawdown of the fully charged batteries (960A total storage). So excess solar can handle some but not all of the hot water heating, depending on how much energy remains in the HWS from the day before, in turn dependent on insulation and how much cold water has been introduced by day-use of hot water. At the moment, we have to remember to turn off the HWS circuit breaker, but we could (like Brian on Insequent) install a 2-3 hour timer on the circuit to avoid running the housebank too low (provided we check the voltage before turning on the HWS charging at the breaker to check the housebank is fully or nearly fully charged by the solar). One alternative is to add more solar and/or more batteries, but that's constrained by real estate as the flybridge is now fully occupied by six solar panels (including one slide out panel from under the radar arch that extends aft over the deck about 0.4 meters). And, in addition to substantial front end cost, albeit zero fuel cost, more PV will not deliver in shitty weather like we are having right now as yet another cataclysmic storm smashes NSW coast (this time south of here, in Sydney and surrounds). But then the genny can be used via AC circuit at fuel-only marginal cost...
3. A diesel heater
such as Webasto hydronic heating system
via an HX loop on Brian's Insequent: this has the co-benefit of offering space heating if one can run the hydronic distribution piping and cabins have accessible space for the radiators); and is likely pretty efficient in using the diesel--my guess is that the system would run at 70-80% delivery of diesel energy content to thermal value of hot water versus using the genny to deliver energy via the AC circuit--which is perhaps 15 max 20% efficient.
Overall: Fairly substantial front end cost, on-going diesel cost and covers cloudy periods w/o further backup needed although genny is always available at higher diesel cost (and noise, fumes...).
4. Last but not least, and the reason for this post: solar thermal
. I have never understood why solar thermal isn't built into production boats, just as PVs are starting to be integrated from get-go either into the topsides or on all sorts of custom mounts seen today on motor vessels and yachts. Obviously, one needs the real estate to mount--but because they collect 80% or more of the solar energy hitting the boat versus max 30-40% for PVs, the footprint is much smaller. At the moment, there are few "marinized" units with fittings suitable for the caustic environment of coastal moorings or at sea, but there are some. I haven't seen any parabolic units on the market with the collector inside a protective cover, but some exist in R&D phase and these are inherently more efficient collectors and could be pole mounted or mounted on a radar arch along with a tracking device. The circulating pump can be solar powered itself w/o a battery that operates at a speed determined by the intensity of the incoming solar energy; or a circulating pump located below decks, presumably in an engine room, powered by an inverter AC circuit or by DC off the housebank.
Here's one such unit produced in the US more for the RV market than the marine market, see
but they make units for marine use specifically
and marketed in Australia
and here's one report from Qld who installed on a yacht before the local distributor setup shop
The units use plumbing for US sized pipes and threads, so that complicates things slightly (the Australian distributor can supply all the substitute parts so you don't have to go searching for the supplier and do all the conversions to get the right units).
Some installation considerations including running the insulated lines to/from the HWS--a 20 meter round trip for hot/cold water on Aquabelle; shading losses just like PVs; and ugliness of visible plumbing on deck or in profile.
Overall, the tradeoffs are the front end cost; finding the real estate (about 2m2 or 4 panels for the Heliatos system sized to our 81 liter HWS); no fuel cost; intermittency of sun so still needs backup for shitty weather, just like the additional PV option, although one could still rely on the genny for these periods.
So there we have it: 4 options at anchor or no shorepower and not under way.
1. Business as usual,
run the genny solely to heat water at marginal cost of fuel, some noxious fumes and noise;*
2. Add PV
to existing PV system to gain more excess power and rely on genny for top-up or backup heating. (Problem, we have no easily available real estate for more PV).
3. Install a diesel heater
, substantial front end cost, always works, but uses diesel albeit about 25% of using genny, so that's a saving relative to BAU; only needs space in ER, no need for deck space to mount but will need it's own exhaust system with less but still present noxious fumes*; has the co-benefit that it can also be used for space heating, but that cost could be avoided at least for distribution components if hot water is sole goal).
4. Install solar hot water
, similar front end cost to 3 but no fuel cost, but does need deck space; and like 2, still needs genny as backup for intermittent sun; but when combined with existing PV to preheat the water, may do the trick most days even in poor solar conditions to minimize backup or top-up genny use.
* an additional consideration; we are moored angled NE to SE at Evans Head. So unlike swinging on an anchor where one is usually head to wind so exhaust streams downwind and away from the boat. cross winds can bring fumes back to the boat and require closure of portholes and windows/doors while genny runs. In summer, this requires turning on the AC while genny runs!
Do any forum members have experience with solar thermal systems on their boat?
Has anyone installed a Heliatos or know of similar units?
Photos of PVs (before the sixth after slide-out panel installed) on hard top are attached for those interested.